New facts from lead-free solders reliability investigation

The lead-free soldering, required by RoHS legislative, has brought many new factors in the solder process as well in reliability and longevity of solder joints, which occur in high quantity in most electronic equipments and systems. In this paper is described a part of the research work, which runs at Brno University of Technology. The aim is to investigate some factors which influencing solder joint reliability and voids restriction, especially by reflow soldering process. The core factor for prediction and evaluation of solder joint reliability is solder joint structure that was monitored on scanning microscope for different solder compositions and processes, where some results were published last year. Our experimental work has indicated that the solder joint structure is strong affected by temperature profile during reflow soldering, especially by speed of the cooling. That is why we have prepared special equipment for cooling to target this parameter for different types of solder materials. Obtained results have shown that the cooling phase has significant effect on solder joint structure formation, especially on creation and forming of intermetalic compounds. That means the cooling phase of the temperature reflow profile has to be setup very carefully to avoid problems with solder joint reliability. Further, we have been leaded new research work to influence the creating of solder joint structure by delivery of supplementary energy in reflow soldering process. First results have shown it could be other way, how to arrange and control better reflow process to assure higher reproducibility and quality of solder joints. Additionally we have tested patterns after thermal cycling by shear test. To make good comparison we studied together with SnPb alloy also SnAgCu (SnAg) and SnCuNiGe system.